Method of producing porcelain enamel coatings

ABSTRACT

A method of applying porcelain enamel to a ferrous enameling sheet stock which permits the use of an electrostatic spray in the method and which further permits eliminating the commonly required application of a ground coat of enamel and the subsequent firing. In this new method the stock is electrostatically sprayed with a flowable enamel material such as a liquid suspended frit to a thickness of a fraction only of the desired final thickness followed by drying this fractional coating slightly until it attains a self-sustaining setup condition which is nonflowing under the conditions to which it is subjected followed by hand spraying over this preliminary coating with a further coating of the enamel material to the desired final thickness and then firing the coatings to flow the enamels to a smooth glassy surface condition. This method is particularly applicable to applying two colors of porcelain enamel to ferrous stock in which one color blends smoothly into the other.

United States Patent Keiser et al.

[ Mar. 7, 1972 [72] Inventors: John E. Keiser, Findlay; Earl Krieger,

Rawson, both of Ohio [73] Assignee: Whirlpool Corporation [22] Filed:May 8, 1970 [21] Appl. No.: 35,610

[56] References Cited UNITED STATES PATENTS 2,828,218 3/1958 Zimmerman..l17/70 C 2,930,713 3/1960 Hoffman ..l17/70C Primary Examiner-Alfred L.Leavitt Assistant Examiner-Edward G. Whitby Anarney.1ames S. Nettleton,Thomas E. Turcotte, Burton H.

Baker, Gene A. Heth, Franklin C. Harter, Anthony Niewyk, Robert L. Juddand l-lofgren, Wegner, Allen, Stellman & Mc- Cord [57] ABSTRACT A methodof applying porcelain enamel to a ferrous enameling sheet stock whichpermits the use of an electrostatic spray in the method and whichfurther permits eliminating the commonly required application of aground coat of enamel and the subsequent firing. In this new method thestock is electrostatically sprayed with a fiowable enamel material suchas a liquid suspended frit to a thickness of a fraction only of thedesired final thickness followed by drying this fractional coatingslightly until it attains a self-sustaining setup condition which isnonflowing under the conditions to which it is subjected followed byhand spraying over this preliminary coating with a further coating ofthe enamel material to the desired final thickness and then firing thecoatings to flow the enamels to a smooth glassy surface condition. Thismethod is particularly applicable to applying two colors of porcelainenamel to ferrous stock in which one color blends smoothly into theother.

9 Claims, No Drawings METHOD OF PRODUCING PORCELAIN ENAMEL COATINGSBACKGROUND OF THE INVENTION Heretofore the use of electrostaticequipment in applying flowable enamel material directly to sheet iron orsimilar stock has not been commercially satisfactory because it wasdifficult to apply the required thickness of the deposit with the resultthat the enamel after firing did not have the desired appearance or wassubject to chipping when stressed. The reason for this was thedifficulty with electrostatic spraying of keeping the total thickness ofthe porcelain enamel at all areas of the metal stock within therelatively narrow limits required by acceptable quality standards tomaintain the desired appearance and to prevent chipping.

In addition, it has always been difficult to produce shaded porcelainenamel and the processes used were laborious and included applying aground coat of enamel material and firing this ground coat before thecolored porcelain enamel could be applied. Furthermore, this coloredporcelain enamel material had to be applied by hand. The handapplication was necessary in order to prevent applying too much enamelmaterial to the article which was of course wasteful of the material andalso hand application was required to prevent producing an inferiorcoating of inferior appearance and excessive susceptibility to damage bychipping.

In order to overcome these problems there have been attempts to applyenamel material electrostatically. These have failed primarily becausethe necessary successive applications of material electrostatically formruns in the material before it had set up and it was very difficult toattain the proper shadmg.

SUMMARY OF THE INVENTION In this invention electrostatic spraying isused but only to apply a fraction of the enamel coating, (which is thenpermitted to dry before) applying the remainder by hand followed bydrying to a self-sustaining state and then firing in the customarymanner.

In this invention the entire novelty is in the method followed. Theenamel materials used are strictly conventional and well understood byenamelers skilled in the art, the type of stock that is used is alsowell understood and the firing temperatures and techniques are likewisewell known.

The ferrous enameling stock which is free from gas forming substancesproductive of pits and spots on subsequent treatment is described inprior publications of which, US. Pat. No. 2,602,758 is a good example.This patent also has a brief description of what is required in a properfiring temperature which, as is explained there, is the temperature atwhich an enamel coating becomes sufficiently fluid to flow on a metalsheet which has been prepared for the enameling to form a smooth glassysurface in an economically short length of time such as 3 to 5 minuteson a -gauge sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of this inventionthe above-described stock is electrostatically sprayed with a fluidenameling material to a fraction of the desired final thickness and theremainder of the enamel material is applied by hand to the desiredthickness. After this has been done the resulting composite coating isdried to a self-sustaining condition after which it is fired to producethe hard glossy enamel as discussed above. As indicated earlier, thismethod which eliminates the need of applying a ground coat followed bythe necessary firing as practiced previously is ideally suited formaking shaded colored enamel such as in producing home appliances. Thusin using the method of this invention in producing such a colored enamelcoating a first area of a ferrous enameling sheet stock as described ishand sprayed with a thin continuous coating of highadherencedark-colored highlighting enamel. Then a second and adjacent area ofthis stock is electrostatically sprayed with a thin continuous coatingof high-adherence light-colored base enamel to a desired thickness whichis a fraction only such as one-half of the desired final thickness.These coatings are then dried slightly to a self-sustaining setupcondition. Next the first area is hand sprayed with highlight materialand the second area of the stock is hand sprayed with a thin continuouscoating of a light-colored base material enamel while substantiallyavoiding overlap onto the first area coating. Then, the first area isagain hand sprayed with a thin continuous coating, of the high-adherencedark-colored highlighting material to overlap the adjacent edge of thesecond area and finally the sheet stock with the enamel coatings thereonis fired as described to flow the enamels to a smooth glassy surfacecondition.

In a specific example of practicing the method of this invention toproduce colored enamel coatings on sheet stock for use in making dryertops a copper colored highlighting enamel and a copper colored baseenamel were used having formulas and physical characteristics asfollows:

Copper lbs. 3 lbs., 12 oz.

Bentonit 1 lb. 14 oz.

Silica 30 lbs. Potassium carbonate 3 lbs., 2 oz. Titanium dioxide #29 3lbs.. 12 oz. Tris nitro 50 grams. Uverite #ZOH. 30 lbs 30 lbs.

Setit HK 1 1b., 14 oz 11b., 4 oz. Chicago Vitreous Brown Oxide #20082.20 lbs 15 lbs. Chicago Vitreous Brown Oxide #20085. 21 lbs., 4 oz. 41lbs., 4 oz. Chicago Vitreous Black Oxide #24492. 12 oz. 6 lbs.. 4 oz.Water 57 gallon. 57 gallon. Fineness oi grind 0-1 gram on 0-1 gram on200 mesh 200 mesh screen. screen. Specific gravity 1.78 1.78.

The ferrous enameling sheet stock which is of the customary type havingedge portions as flanges and the remainder as the face of the resultingenameled sheet was treated as follows:

Air Fluid Thick- Speclfic Pick pressure, pressure, ness Step Enamelgravity up lbs. lbs. Wei 1 Highlight- 1.78 120 15 70 .005 2..- se 1.6365 7 45 .006 3 Highlight. 1.78 15 60 .003 4.-. Base 1.78 15 70 .004 5Highlight- 1.76 70 5 70 .002

Pick up-the amount of enamel retained on a 1 sq. ft. cylinder (ingrams).

The above Base and Highlight" formulas are themselves strictlyconventional and well known.

In Step 1 the edges of the sheet corresponding to the flanges in theresulting dryer panel were hand sprayed with the highlight enamel havingthe characteristics given. In Step 2 the face of the sheet or the areawithin the boundary flanges was electrostatically sprayed with the baseenamel to one-half the desired final thickness. Following Step 2 thethusly applied enamel coats were dried slightly until the coatings wereselfsustaining. In this embodiment, as indicated, the drying time underheat lamps was for only 30 seconds. Then, in Step 3, the flanges wereagain hand sprayed with highlight material to the thickness indicated.In Step 4 the face was again sprayed with base material but this timewas hand sprayed rather than electrostatically sprayed and the oversprayaround the edges was kept to a minimum which meant that there wassubstantially no overlap. in Step 5 the highlight material was againhand sprayed over the flanges and to overlap the adjacent edge of theface. In each instance the above table gives gravity, the air pressure,the fluid pressure and the thickness in fractions of an inch of the wetcoating.

As indicated by the line speed of 23 feet per minute, these panels wereadvanced continuously in the customary manner during the indicatedprocessing. After all the enamel had been deposited on the panels thepanels were dried as indicated in an oven for 2 minutes at 375 F. Asshown in the above table the final firing was in a furnace havingsuccessive zones with different and generally increasing temperaturesthrough which the panels passed.

Having described our invention as related tothe embodiments set outherein, it is our intention that the invention be not limited by any ofthedetails of description, unless otherwise specified, but rather beconstruedbroadly within its spirit and scope as set out in the appendedclaims.

We claim:

1. The method of making a smooth coating of porcelain enamel withoutrequiring application of a ground coat and the subsequent firingthereof, comprising: electrostatically spraying a ferrous enamelingsheet stock free from gas forming substances productive of pits andspots with a preliminary coating of an enamel material to a thickness ofa fraction only of the desired final thickness; drying said coatingslightly to a self sustaining setup condition; hand spraying saidpreliminary coating with a further coating of an enamel material to adesired final thickness; and firing said coatings to flow the enamels toa smooth, glassy surface condition.

2. The method of claim 1 wherein said electrostatic spraying is to athickness of 40 to 60 percent of said desired final thickness.

3. The method of claim 1 wherein said electrostatic spraying is to a wetthickness of 0.004 to 0.006 inch and said hand spraying is to athickness of 0.004 to 0.006 inch.

4. 'The method of making shaded smooth coatings of two superimposedcolors of porcelain enamel without requiring application of a groundcoat and the subsequent firing thereof,

comprising; as a first step hand spraying a first area of a ferrousenameling sheet stock free from gas forming substances productive ofpits and spots with a thin continuous coating of high-adherencedark-colored highlighting enamel; as a second step electrostaticallyspraying a second and adjacent area of said stock with a thin continuouscoating of high-adherence light-colored base enamel; as a third stepdrying the resulting coatings slightly to a self-sustaining setupcondition;' as a fourth step hand spraying said second area of saidstock with a thin continuous coating of high-adherence light-coloredbase material enamel while substantially avoiding overlap onto saidfirst area coating; as a fifth step again hand spraying said first thespecific area with a thin continuous coating of high-adherencedarkcolored highlighting-enamel and to overlap the adjacent edge of saidsecond area; and firing said coatings to flow the enamels to a smooth,glassy surface condition.

5. The method of claim 4 wherein said first-step spraying is to a wetthickness of 0.004 to 0.006 inch, said second-step spraying is to a wetthickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wetthickness of 0.004 to 0.006

inch, and said fifth-step spraying is to a wet thickness of 0.004 to0.006 inch.

6. The method of claim 5 wherein said firing temperature is about l,420to 1480" F.

7. The method of making shaded smooth coatings of two superimposedcolors'of porcelain enamel without requiring application of a groundcoat and the subsequent firing thereof, comprising: as a first step handspraying a first area of a ferrous enameling sheet stock free from gasforming substances productive of pits and spots with a thin continuouscoating of high adherence dark colored highlighting enamel; as a secondstep electrostatically spraying a second and adjacent area of said stockwith a thin continuous coatingof high-adherence light-colored basematerial enamel; as a third step drying the resulting coatings slightlyto a self-sustaining setup condition; as a fourth step hand sprayingsaid first area of said stock with a thin continuous coating ofhigh-adherence dark-colored highlighting enamel; as a fifth step-handspraying said second area of said stock with a thin continuous coatingof high-adherence light-colored base material enamel while substantiallyavoiding overlap onto said first area coating; as a sixth step againhand spraying said first area with a thin continuous coating ofhigh-adherence dark-colored highlighting enamel; and firing saidcoatings to fiow the enamels to a smooth, glassy surface condition.

8. The method of claim 7 wherein said first-step spraying is to a wetthickness of 0.004 to 0.006 inch, said second-step spraying is to a wetthickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wetthickness of 0.004 to 0.006 inch, said fifth-step spraying is to a wetthickness of 0.004 to 0.006 inch, and said sixth-step spraying is to awet thickness of 0.001 to 0.003 inch.

9. The method of claim 8 wherein said firing temperature is about 1,420to l,460 F.

2. The method of claim 1 wherein said electrostatic spraying is to athickness of 40 to 60 percent of said desired final thickness.
 3. Themethod of claim 1 wherein said electrostatic spraying is to a wetthickness of 0.004 to 0.006 inch and said hand spraying is to athickness of 0.004 to 0.006 inch.
 4. The method of making shaded smoothcoatings of two superimposed colors of porcelain enamel withoutrequiring application of a ground coat and the subsequent firingthereof, comprising: as a first step hand spraying a first area of aferrous enameling sheet stock free from gas forming substancesproductive of pits and spots with a thin continuous coating ofhigh-adherence dark-colored highlighting enamel; as a second stepelectrostatically spraying a second and adjacent area of said stock witha thin continuous coating of high-adherence light-colored base enamel;as a third step drying the resulting coatings slightly to aself-sustaining setup condition; as a fourth step hand spraying saidsecond area of said stock with a thin continuous coating ofhigh-adherence light-colored base material enamel while substantiallyavoiding overlap onto said first area coating; as a fifth step againhand spraying said first area with a thin continuous coating ofhigh-adherence dark-colored highlighting enamel and to overlap theadjacent edge of said second area; and firing said coatings to flow theenamels to a smooth, glassy surface condition.
 5. The method of claim 4wherein said first-step spraying is to a wet thickness of 0.004 to 0.006inch, said second-step spraying is to a wet thickness of 0.004 to 0.006inch, said fourth-step spraying is to a wet thickness of 0.004 to 0.006inch, and said fifth-step spraying is to a wet thickness of 0.004 to0.006 inch.
 6. The method of claim 5 wherein said firing temperature isabout 1,420* to 1480* F.
 7. The method of making shaded smooth coatingsof two superimposed colors of porcelain enamel without requiringapplication of a ground coat and the subsequent firing thereof,comprising: as a first step hand spraying a first area of a ferrousenameling sheet stock free from gas forming substances productive ofpits and spots with a thin continuous coating of high adherence darkcolored highlighting enamel; as a second step electrostatically sprayinga second and adjacent area of said stock with a thin continuous coatingof high-adherence light-colored base material enamel; as a third stepdrying the resulting coatings slightly to a self-sustaining setupcondition; as a fourth step hand spraying said first area of said stockwith a thin continuous coating of high-adherence dark-coloredhighlighting enamel; as a fifth step hand spraying said second area ofsaid stock with a thin continuous coating of high-adherencelight-colored base material enamel while substantially avoiding overlaponto said first area coating; as a sixth step again hand spraying saidfirst area with a thin continuous coating of high-adherence dark-coloredhighlighting enamel; and firing said coatings to flow the enamels to asmooth, glassy surface condition.
 8. The method of claim 7 wherein saidfirst-step spraying is to a wet thickness of 0.004 to 0.006 inch, saidsecond-step spraying is to a wet thickness of 0.004 to 0.006 inch, saidfourth-step spraying is to a wet thickness of 0.004 to 0.006 inch, saidfifth-step spraying is to a wet thickness of 0.004 to 0.006 inch, andsaid sixth-step spraying is to a wet thickness of 0.001 to 0.003 inch.9. The method of claim 8 wherein said firing temperature is about 1,420*to 1,460* F.